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Not being able to see planets as the sun is too bright?

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  • 12-10-2020 8:13pm
    #1
    Smiles35
    Registered Users Posts: 1,749 ✭✭✭


    I've been reading that current telescopes can't see planets outside our solar system as their host star ''is too bright''. It's a resoulution issue though really isnt it?


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    #2
    Larbre34
    Registered Users Posts: 23,797 ✭✭✭✭Larbre34


    Smiles35 wrote: »
    I've been reading that current telescopes can't see planets outside our solar system as their host star ''is too bright''. It's a resoulution issue though really isnt it?

    No, its not a resolution issue. Exoplanets, or planets orbiting stars other than our own are simply too dim in pretty much all circumstances to be observed visibly from our solar system, even with space based devices. You'll have seen some of the Hubble images of deep space showing galaxy clusters, the clouds of nebulae and so on. Imagine trying to identify visually a single planet perhaps many trillionths the size of what appears already as a small object in those images.

    The behaviour of the star itself may tell us, within a wide range of tolerance, whether it has planets in its orbit. But our technology can really only detect giant planets influencing the movement of their star. Planets the size of Earth, without a fellow giant planet in their system, do not generally produce enough movement on the star on their own for us to know they are there.


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    #3
    Smiles35
    Registered Users Posts: 1,749 ✭✭✭Smiles35


    Larbre34 wrote: »
    No, its not a resolution issue. Exoplanets, or planets orbiting stars other than our own are simply too dim in pretty much all circumstances to be observed visibly from our solar system, even with space based devices. You'll have seen some of the Hubble images of deep space showing galaxy clusters, the clouds of nebulae and so on. Imagine trying to identify visually a single planet perhaps many trillionths the size of what appears already as a small object in those images.

    That's pretty much what I'm thinking. So, it's not an issue per se. It's the fact we would have to build something a fraction of the size of the earth to get a solid view.
    Larbre34 wrote: »
    The behaviour of the star itself may tell us, within a wide range of tolerance, whether it has planets in its orbit. But our technology can really only detect giant planets influencing the movement of their star. Planets the size of Earth, without a fellow giant planet in their system, do not generally produce enough movement on the star on their own for us to know they are there.

    What are we going to do so? If we are ever going to go to the trouble of launching probes it will depend on us being sure of an oxigen rich world to shoot towards. I supose the test of what can be achieved will be a look at the 4 light year away proxima with our up and coming best. The JWST next year and the ELT, five years away.

    This talk of putting sun shields on telescopes to combat brightness seems like snake oil to me.


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    #4
    mikhail
    Registered Users Posts: 3,863 ✭✭✭mikhail


    A system like a telescope can be described by its point spread function. A point of light, imaged, produces something smeared out. If you have two points close by each other, that smearing tends to cause them to overlap. If one is much dimmer than the other, it is no longer visible. If we have two points of equal brightness, there's a minimum distance between them when we can still see two distinct peaks. That distance is one widely used definition of resolution, called the Rayleigh criterion. So you're right, it's an issue of resolution, but that's not to say it's not also an issue of relative brightness.

    You may find this helpful: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/Raylei.html

    I've never heard of sun shields in this context though. The James Webb has a sun shield, but it's for protecting it from the thermal effects of our sun, not the light from object stars.


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    #5
    Larbre34
    Registered Users Posts: 23,797 ✭✭✭✭Larbre34


    Smiles35 wrote: »
    That's pretty much what I'm thinking. So, it's not an issue per se. It's the fact we would have to build something a fraction of the size of the earth to get a solid view.



    What are we going to do so? If we are ever going to go to the trouble of launching probes it will depend on us being sure of an oxigen rich world to shoot towards. I supose the test of what can be achieved will be a look at the 4 light year away proxima with our up and coming best. The JWST next year and the ELT, five years away.

    This talk of putting sun shields on telescopes to combat brightness seems like snake oil to me.

    We can't get a visual of these planets, full stop. And to send a probe to Proxima b planet, the nearest known planet to our solar system would take 6,000 years and 7 year minimum to transmit data, assuming everything went perfectly. So what would be the point.


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    #6
    Smiles35
    Registered Users Posts: 1,749 ✭✭✭Smiles35


    Larbre34 wrote: »
    We can't get a visual of these planets, full stop. And to send a probe to Proxima b planet, the nearest known planet to our solar system would take 6,000 years and 7 year minimum to transmit data, assuming everything went perfectly. So what would be the point.

    6000? I heard of estimates of getting something up to 10% of the speed of light. 4 light years to Proxima B. That's 40 years.

    It's the next challenge. Ten mass drivers on the moon or something.


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    #7
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    Smiles35 wrote: »
    I've been reading that current telescopes can't see planets outside our solar system as their host star ''is too bright''. It's a resoulution issue though really isnt it?

    You don't have to be able to resolve an object in order to see it. If you did, we wouldn't be able to see any stars in the night sky as they are all way below the resolution of the human eye. Even telescopes can only resolve the larger / closer ones. We can still get useful information from unresolved objects using spectroscopy. The main thing is to be able to resolve the planet's separation from the host star, but that's well within the diffraction limit of large telescopes even at tens of light years.

    It's not true that current telescopes can't see exoplanets -- several dozen have been directly imaged so far. But it is true that host star brightness is a limiting factor which is why almost all the imaged planets are at least 10 AU from the host, many much further. Time integrated imaging can capture incredibly faint objects but only, of course, if the signal is above the noise level and not swamped by something brighter. All of the techniques for direct imaging involve somehow subtracting the image of the host, either by occlusion (various types of aperture masking) or using an image of a similar reference star to figure out what you don't want to capture (differential imaging).

    Even at that, most are a bit of a cheat in that they are not imaged in the reflected light of the host star the way you might expect. They are young planets, still glowing in their own energy of formation and can only be seen in the infrared. In reflected visible light, a planet like Jupiter is over a billion times fainter than our Sun and at, say, 70 light years, its angular separation from the Sun would be half an arc second -- the same as the width of a euro two cent coin seen from five miles away. But as mentioned, that's easily within the diffraction limit of a 10-metre class telescope so yes, it's really a brightness issue.

    Nevertheless, astronomers recently managed to image a similar planet in visible light. It was done for the planet β Pictoris b in 2014. The host star is 63 light years away and about nine times the luminosity of the Sun, and the planet's orbit is about the size of Saturn's. Once again there's a slight cheat involved as the planet had already been imaged in infrared, and two cameras were used simultaneously so that the visible light image could be confirmed by comparison to the longer wavelength one.

    Future ideas for direct imaging include the "starshade" idea -- flying two spacecraft 25,000 miles apart with one carrying a camera and the other an umbrella to block off the light from the host star in a stellar system. It doesn't actually work quite like an umbrella. At such a tiny angular width, light from the star easily diffracts around the starshade. The shade is actually a precise petal arrangement such that the diffracted rays cancel each other out by interference at the imaging camera. Pretty neat!


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